The present invention relates to hydraulic motors of the gerotor type, and more particularly, to a parking brake for use in such motors.
Although the present invention may be utilized in hydraulic devices functioning as pumps, it is especially advantageous when used with a device operating as a motor, and will be described in connection therewith.
Hydraulic motors utilizing gerotor displacement mechanisms have been popular for many years for low speed high torque applications. Such motors typically include a housing defining a fluid inlet and a fluid outlet and a gerotor gear set associated with the housing. The gerotor gear set normally includes an internally-toothed ring fixed to the housing, and an externally-toothed star eccentrically disposed within the ring for orbital and rotational movement relative to the ring. The teeth of the ring and star interengage to define expanding and contracting volume chambers during the relative movement. A valve means within the housing operates in response to the relative movement to communicate fluid from the fluid inlet to the expanding fluid chambers, and from the contracting fluid chambers to the fluid outlet. An output shaft extends from the housing and is rotatably supported thereby, and a shaft member has a first end connected to the star and a second end connected to the output shaft, to transmit the orbital and rotational movements of the star into a low speed high torque rotational movement of the output shaft.
As is well known to those skilled in the art, various arrangements of gerotor motors are known other than that described above wherein the ring is fixed to the housing and the star orbits and rotates, and although the invention may be applied advantageously to such other gerotor motor arrangements, as will be described subsequently, the above-described arrangement is the most common and constitutes the preferred embodiment of the invention.
Low speed high torque gerotor motors are frequently used to propel the drive wheels of vehicles, thus making it desirable for the motor to include some form of parking brake. Another major use of such motors is to drive vehicles accessories, such as hoists and winches, and in this type of application it is desirable for the motor to have a "load holding" capability.
One attempt by those working in the prior art to provide a motor equipped with a parking brake is shown in U.S. Pat. No. 3,616,882 in which a thin, flexible friction member can be selectively pressure biased into engagement with an end face of the gerotor star. However, it appears that such an arrangement would result in excessive friction and generated heat, with the possibility of galling the end surface of the star. In addition, the parking brake shown in the cited reference is not positive acting, but instead, depends upon the presence of hydraulic pressure.
Another approach to the need for a positive acting brake has been the use of a disc pack, with some of the discs being splined to the fixed housing, and alternating discs being splined to the rotating output shaft. Typically, the discs are spring biased into engagement (braking) and hydraulic pressure is required to disengage the discs. In one commercial embodiment, the disc pack is disposed within the housing of the gerotor motor and is operable to lock the motor output shaft to the motor housing. This approach requires almost total redesign of the motor housing and output shaft, thus making it economically impractical to offer a parking brake as a motor option. In another commercial design, a separate parking brake package engages the motor output shaft and has its own housing and separate output shaft which can be locked together by engagement of a disc pack. This separate parking brake has the advantage that it can be added as an option, because no major modification of the motor is required, but the cost of the commercially available parking brake may be as much or more than the gerotor motor itself.